A number of endocrine factors have been found to influence the development of plasmacytoma in the pristane-injected BALB/c mice. The discovery of two non-endocrine factors, bacterial lipopolysaccharide and pristane-induced peritoneal factors, has led to the development of an hypothesis integrating the profound effects of certain endocrine agents on the development of this tumor. Bacterial lipopolysaccharide administration increases the development of tumors in pristane-injected mice at some early step which produces nascent tumors, which cannot readily be transplanted to syngeneic mice. Intraperitoneal administration of pristane produces a factor in peritoneal fluid within three days which allows ready transplantation of primary tumors to appropriate hosts. This material contains no pristane. It is proposed to relate the endocrine factors that either accelerate or prevent tumorigenesis to the increase in peritoneal cells and bacterial lipopolysaccharide produced by intraperitoneal pristane, to a transformation step, to nascent tumor cells, or to a maturation step induced by our pristane-induced peritoneal factor.